Heretofore, little attention has been given to the formation of low magnification photographic images with electron microscope. However there is a growing demand to be able to compare the image of a specimen obtained with an optical microscope, which has therefore a relatively low magnification and wide field of view, with a corresponding image obtained with an electron microscope. In order to obtain the latter image it is necessary to use, in the electron microscope, a relatively thick specimen of the nature suitable for optical microscope. However, it is very difficult with a conventional electron microscope to produce an image of uniform high quality on a photographic plate, even if the specimen to be examined is ultra thin, and needless to say even more so in the case of a relatively thick specimen. The reason for this being: (1) the electron microscope is subject to several aberrations which are impossible to compensate for, since the usual electron microscope is provided with only converging lens, (2) the aberrations become abruptly noticeable when the converging lenses are used with low gain in magnification, and (3) the amount of inelastic scattering electrons increases with the increment thickness of the speciment to be examined. Further, the scattering angles thereof increase so that the circumferential portion of the image focused on the photographic plate is blurred. Therefore, the image photographed with low magnification has a satisfactory definition only at its center portion. Furthermore, with the conventional electron microscope, divergence of the electron beam is limited by the pole pieces of the electron lenses or diameter of a fixed aperture, so that only a small portion of the object can be photographed at the same time even if the magnification is lowered to a considerable extent. Therefore, in order to obtain a photograph of a wide field range of object while maintaining satisfactory quality, it has been necessary to assemble or neighbor a plurality of photographs of somewhat high magnification. Such a case, however, requires a tremendous amount of cumbersome work, and encounters an additional problem that the plurality of photographs can not be assembled precisely due to distortion of the electron lenses as well as the shrinkage and/or expansion of photographic paper.
It is therefore an object of this invention to provide a method of photographing images of different portions of a single photographic plate using an electron microscope, and apparatus therefor, by which a high and uniform quality, wide field of view image of low magnification can be recorded on the single photographic plate.